Public-address system



June 11, 1929. E. MELHUISH PUBLIC ADDRESS SYSTEM Filed Nov. 26, 1927 Nll IHL nu \N .QN ww I 5% 1 ww j bw www .i E ll M m /NVENTUR LAURENCE EMELHll/SH BY ATT RNEY of this invention each the medium of the IPatented June 11, 1929.

l,7li,055

UNITED STATES PATENT OFFICE.

LAWRENCE E. MELHUISH, OF GLEN RIDGE, NEW J'ERSEY, ASSIGNOR TO BELL TELE-PHONE LABORATORIES, INCORPORATED, OF NEW NEW YORK.

YORK, N. Y., A. CORPORATION OF PUBLIC-ADDRESS SYSTEM.

Application filed November 26, 1927. Serial No. 235,812.

This invention relates to switching systems and is particularlyapplicable to signaling, general announcing, and public ad dresssystems. 1

In such systems it frequently becomes desirable to connect all of theloud speakers or other signaling devices in circuit at one time forvarious. purposes such as the making of general announcements.Heretofore as been possible to accomplish this result only by the use ofelaborate and expensive relay systems. It is the object of thisinvention to provide a cheap, simple and uni tary switc ing meanswhereby such a switching operation may be accomplished by the actuationof one switch key, regardless of the position of the individual circuitcontrols. d p

In accordance with the general features individual or group circuit keyis provided with back contacts connected to a common bus.v A doublethrow master key in itsnormal position permits the group circuits to becontrolled individually but when thrown in the emergency position itbrings into circuit through common bus any and all of the speakers notalready in circuit.

Another feature of this invention ismeans under the control of themaster key for preventing clicks in the loud speakers due tointerruptions of the transmitter current.

Both of these features are illustrated in the embodiment of thisinvention shown on the accompanying drawing. This switching system isconsiderably simplified by the use of diagrammatic symbols for certainapparatus which in itself forms no part of the present invention, forinstance, a rectangle containing the letter A is used to denote acomplete vacuum tube amplifier. Only a few group circuit switches areshown but this number obviously could be increased to meet therequirements of any particular installation.

As indicated by the disposition of the apparatus in the drawing, theswitching keys and the microphone are arranged to be located in oneplace and the amplifiers, relays, etc. in another. This remote controlfeature is often of considerable importance as, for instance, when thesystem is used in hospitals or other institutions, and placed,

under the control of some one person such other cases it may within thereach of the operator while the amplifiers, batteries, etc.

placed in the basement or any other suitable place. The number andlocation of the various individual loud speakers Wlll of course dependupon the size and nature of the particular institution wherein thesystem is used. In general, however, the several speakers necessary forone large room, or for a number of rooms used for similar purposes, maybe connected to one operatedas a unit. In be desirable to provideseparate keys for. a number of individual loud speakers.

When it is desired to put the system into operating condition the masterkey K is moved to its downward or normal operating position. Contact 1of this key completes the secondary circuit of a supply transformer Twhich is used to transform the 110 volt alternating current suppliedfrom source G to a. lowervoltage suitable for relay operation such as 24volts. Current from the secondary winding of transformer T traversesconductor 2, contact 1, conductor 3, windings of relays 4 and 5 inparallel to transformer again and actuates the relays.

The operation of relay 4 closes contact 6 which completes circuits forthe microphone M and the filaments of amplifier A The microphone is ofthe push-pull type with conductors and 47 permanently connected to thespeech input transformer 48. lVhen contact 6 closes, battery is suppliedto the speech circuit over conductors 51, 9 and 46 and to the filamentsover conductors 10, 9 and 44.

In closing, contacts 11 and 12 of relay 5 impress 110 volts alternatingcurrent upon conductors l4 and 15 leading to filament transformer 18 ofamplifier A and energizes a slow acting relay 13. Filament transformer19 of rectifier 52 is also energized over conductors 7 and 8.

After a delay of several seconds slow acting relay 13 closes itscontacts and completes the circuit to the plate voltage supplytransformer .20 of amplifiers A and A from conductors 14 and 15 overconductors 8, 60 and 17. This delay is necessary to protect contacts 26and ably result if plate voltages were applied to them before theirfilaments had become properly heated.

The secondary of transformer 20 is connected by conductors 53 and 54 toa full wave rectifier 52 which is arranged to impress its high voltagedirect current output across the potentiometer 55. Proper plate voltagesare obtained by tapping potentiometer 55 at points 57, 58 and 59.

It should be noted that both the filament and plate circuits of theamplifiers A and A have been closed so that the system is now ready foroperation. This information is conveyed to the operator by the glowingof indicating lamp L which is lighted by the closing of contact 21 ofrelay 13. If the operator then desires to page some person believed tobe in'the neighborhood of the loud speakers controlled by key 25, forinstance, he will actuate that key to close its 27. He will then addressthe message to the microphone M and the voice currents will be deliveredto the preliminary amplifier A over the circuit previously traced,amplified and impressed upon final amplifier A over conductors 21 and 22for further amplification in the well known manner. The output of thefinal amplifier will then actuate the loud speakers 30 and 31 previouslyselected over a circuit comprising conductors 23 and 24, contacts 26 and27 and conductors 28 and 29. In a similar .manner any other groupcircuit or single speaker could be operated by actuating the proper keysuch as 32, 33 or 34.

Sometimes it may be desirable to make a general announcement or to sendout an alarm. When the number of group circuits is large it wouldinvolve considerable effort and delay to deliver the message to eachcircuit separately, accordingly the master key K is arranged to connectall of the loud speakers in circuit at one time when thrown to itsemergency or upward position. Normally closed'back contacts of eachgroup switch key, such as contacts 35 and 36 of key 25 are connected toa common bus consisting of conductors 37 and 38 which ter minate on theemergency side of master key K. lVhen the key is thrown to the emer--gency position the output of amplifier A actuates all loud speakers overa circuit extending from the amplifier over conductors 23 and 24,contacts 39 and 40 of key K, common buses 37 and 38, back contacts ofeach group key, conductors 28 and 29, 42 and 43, etc. to each loudspeaker. If at the time such a general announcement is made one or moreof the group keys have been left in the operated position thecorresponding speakers will nevertheless be operated simultaneously withthe others over the normal circuit including conductors 23 and 24.

When the system is not required for immediate use it may be renderedinoperative and completely deenergized by simply returning the masterkey to its neutral or off position. When this is done the secondarycircuit of supply transformer T is broken at contact 1 or 41 dependingupon the position in which the key was last set. Indicating lamp L isthereupon extinguished and relays 4 and 5 contacts 11 and 12 of relay 5disconnect filament supply'transformers 18 and 19 and plate supplytransformer 20,'and release relay 13.

' Relay 4 is slow in releasing so that contact 6 is not broken until '5to 10 seconds after the plate voltage supply transformer is deenergized.This delay in opening of the transmitter circuit until after the platecircuits are deenergized prevents the click, 1

produced when the battery current is interrupted, from being heard inthe loud speakers.

A particular application of the features in this invention has beendescribed by way.

of illustration. It is not intended, however,

to limit the invention by this illustration but only according to thescope of the appended claims.

What is claimed is: 1. In a public address system, a speech inputcircuit, a speech output circuit, a plurality of loud speaking devices,switching means individual to said devices, other switching meansarranged to connect in said output circuit all of said devicessimultaneously regardless of the position of said individual switchingmeans, and means responsive to said other switching means formaintaining said input circuit until after said output circuithas beenopened.

' 2. In a public address system, a speech input circuit, a speech outputcircuit, a plurality of loud speaking devices, switching meansindividual to the devices, other switching means arranged to connect inthe output circuit all of the devices simultaneously, regardless of theposition of the individual switching means, and a slow releasing relayunder the control of the said other switching means for maintainingtheinput circuit until the output circuit has been opened.

a In witness whereof, I hereunto subscribe my7name this 22 dayofNovember, A. D. 192

LAWRENCE EJ-"MELHUIs

